Igneous Rock Types

Why This Matters

Igneous rocks are the foundation of the rock cycle—literally where it all begins. When you understand how these rocks form, you're unlocking the keys to plate tectonics, volcanic activity, and crustal composition. The AP exam loves to test whether you can connect a rock's texture to its cooling history, or predict what kind of igneous rock forms at a specific tectonic setting. You're being tested on your ability to read a rock like a story: texture tells you cooling rate, composition tells you magma chemistry, and location tells you tectonic context.

Don't just memorize rock names and colors. Know why granite has large crystals while basalt has tiny ones. Understand how silica content controls viscosity and eruption style. When you can explain the mechanism behind each rock type, you'll crush both multiple-choice questions and FRQs that ask you to compare formation environments or predict rock properties from given conditions.

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Intrusive (Plutonic) Rocks: Slow Cooling Below the Surface

When magma cools slowly deep within Earth's crust, crystals have time to grow large and visible. The slower the cooling, the larger the crystals—this is the key principle behind all coarse-grained igneous rocks.

Granite

• Coarse-grained texture with visible crystals of quartz, feldspar, and mica—the classic example of slow cooling in plutonic environments
• Felsic composition means it's rich in silica and light in color, forming from magma with high viscosity
• Continental crust foundation—granite batholiths form the cores of mountain ranges and are extremely durable

Gabbro

• Coarse-grained and dark-colored, composed mainly of plagioclase feldspar and pyroxene—the intrusive equivalent of basalt
• Mafic composition with high iron and magnesium content gives it greater density than granite
• Forms oceanic crust in the lower layers and upper mantle, making it critical for understanding seafloor structure

Diorite

• Intermediate composition between granite and gabbro, often displaying a distinctive salt-and-pepper speckled appearance
• Contains plagioclase feldspar and hornblende—the mix of light and dark minerals reflects its chemical middle ground
• Forms in continental crust from slowly cooling magma, often found alongside granite in plutonic complexes

Pegmatite

• Exceptionally large crystals—some measuring meters across—formed during the final stages of magma crystallization
• Water-rich magma pockets allow ions to migrate freely, producing giant crystal growth in granite margins
• Source of rare minerals including lithium, beryllium, and gemstones like tourmaline and aquamarine

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Extrusive (Volcanic) Rocks: Rapid Cooling at the Surface

When lava erupts and cools quickly at Earth's surface, crystals have little time to form. Rapid cooling produces fine-grained textures or even glass—the opposite of what happens underground.

Basalt

• Fine-grained, dark-colored, and mafic—forms from low-viscosity lava that flows easily across the surface
• Dominates the ocean floor and creates volcanic islands, shield volcanoes, and flood basalt provinces
• Extrusive equivalent of gabbro—same composition, completely different texture due to cooling rate

Rhyolite

• Fine-grained and felsic, rich in silica with quartz and feldspar—the extrusive equivalent of granite
• High-viscosity lava traps gases and leads to explosive volcanic eruptions rather than gentle flows
• Associated with continental volcanism and caldera-forming events like Yellowstone

Andesite

• Intermediate composition between basalt and rhyolite, typically gray to brown with porphyritic texture
• Subduction zone signature—commonly found in volcanic arcs where oceanic crust dives beneath continental crust
• Porphyritic texture with larger crystals in a fine matrix indicates two-stage cooling: slow at depth, then rapid at surface

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Volcanic Glass and Vesicular Textures: Extreme Cooling Conditions

Some volcanic rocks form under such rapid or gas-rich conditions that they develop unique textures—glassy surfaces from instant cooling or vesicles (holes) from trapped gas bubbles.

Obsidian

• Volcanic glass formed when silica-rich lava cools so rapidly that no crystals can form
• Conchoidal fracture produces razor-sharp edges, making it historically valuable for tools and weapons
• Felsic composition despite its dark color—the glassy texture masks what would otherwise appear light-colored

Pumice

• Highly vesicular and low-density—the only rock that commonly floats on water due to abundant trapped gas bubbles
• Forms during explosive felsic eruptions when frothy, gas-charged magma rapidly depressurizes and solidifies
• Used as an abrasive in industrial and cosmetic applications because of its rough, porous texture

Scoria

• Vesicular and mafic, typically reddish-brown or black—the basaltic equivalent of pumice
• Denser than pumice because mafic lava has lower gas content and larger, less numerous vesicles
• Common in cinder cones and often used as lightweight landscaping aggregate

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Quick Reference Table

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Self-Check Questions

1. Texture comparison: Both granite and basalt can have similar mafic or felsic compositions to their counterparts—what single factor explains why granite is coarse-grained and basalt is fine-grained?

2. Identify by concept: Which two rocks would you expect to find at a mid-ocean ridge, and what roles do they play in oceanic crust structure?

3. Compare and contrast: How do pumice and scoria demonstrate the relationship between magma composition, gas content, and rock density?

4. Tectonic connection: If you found andesite with porphyritic texture at a volcanic arc, what does this tell you about both the tectonic setting and the cooling history of the magma?

5. FRQ practice: Explain why rhyolite and granite have the same chemical composition but form under different conditions. How would you use these two rocks to illustrate the difference between intrusive and extrusive igneous processes?